In optical recording disks, magnetooptic recording disks, and optical read only disks, various grooves and pits are formed as recording spots and for tracking and other addressing purposes. Such optical recording disks, magnetooptic recording disks, and optical read only disks are manufactured by either an injection method in which grooves and pits are formed at the same time as injection molding of a substrate or a so-called 2P method in which a photo polymer layer is formed on a substrate and grooves and pits are formed in the photo polymer layer. In either method, disks are manufactured in a reproducible manner using a stamper for duplicating the grooves and pits.
The stamper is produced by the following sequence of steps:
(1) abrading one surface of a substrate, for example, a disk-shaped glass plate which forms an original for stamper production into a flat surface,
(2) scrubbing the abraded surface,
(3) cleaning the abraded surface after scrubbing,
(4) forming a photoresist layer on the abraded surface,
(5) exposing the photoresist layer to light, for example, laser beams in a mother pattern corresponding to a stamper pattern for duplicating grooves and/or pits in optical disks,
(6) developing the photoresist layer after exposure, to thereby form the mother or original pattern,
(7) forming an underlying metal layer on the mother pattern,
(8) electroforming an overlying metal layer on the underlying layer,
(9) removing the metal film of overlying and underlying metal layers from the glass plate, the metal film having a surface for duplicating the mother pattern and a rear surface,
(10) shaping the outer and inner peripheries of the metal film,
(11) polishing the rear surface of the metal film into a flat surface, and
(12) other necessary processing.
Since the performance of optical disk depends on the flatness accuracy of the surface in which grooves and/or pits (grooves being often used as a representative term, hereinafter) are formed, it is a critical factor in the manufacture of optical disks to increase the flatness of the front surface of a glass plate which is a substrate for the production of a stamper and the rear surface of the stamper.
The flatness of the rear surface of a stamper directly governs the depth and width of grooves in the resulting optical disks because the stamper is set in place within a mold in the injection molding method or the stamper is attached to the pressing surface of a stamping machine in the 2P method. Therefore, attempts have been made to increase the flatness of the rear surface of a stamper by polishing or the like. The attempts for polishing the stamper rear surface include the use of free abrasive grains and the use of abrasive tape. The former is known as a free abrasive method. The latter method laps the rear surface of the stamper by rotating the stamper while pressing abrasive tape against the rear surface of the stamper through a pressure member such as a rubber roller as disclosed in Japanese Patent Application Kokai No. 196962/1983.
In optical disks, information is reproduced by directing reproducing light, typically laser light from a reproducing head to the information recording or carrying surface. If the information carrying surface has irregularities, the reproducing head is accelerated upward or downward for focusing. The acceleration, known as dynamic axial runout acceleration, is increased as the disk revolution is increased. For example, compact disks (CD) which are typical optical read only disks are rotated about 600 r.p.m. at the maximum during operation, whereas information recording optical disks like magnetooptic recording disks which require quick access and data transfer are operated at as high revolution as 1,800 to 3,600 r.p.m. The dynamic axial runout acceleration at a location spaced 55 mm radially from the disk center during operation at 1,800 r.p.m., for example, is about 8 times the CD operation. It is desired to minimize irregularities on the disk substrate surface particularly in information recording optical disks adapted for high speed rotation. To this end, the stamper rear surface should be polished to a more precise accuracy.